I assume the sound waves overlap and increase in amplitude, destroying the tumor cells. But I'm wondering why a shrinking tumor is enough - usually we cut the

I assume the sound waves overlap and increase in amplitude, destroying the tumor cells. But I'm wondering why a shrinking tumor is enough - usually we cut the whole thing out so that it doesn't metastasise or regrow from small leftover parts. If the beam kills most of it, is that enough? Does it trigger a mass die-off or stimulate our immune system to do the rest of the job? Just wondering what the explanation here is.